Shearography is method for nondestructive inspection used to detect surface deformation. It is conducted by illuminating a surface of a test object using laser light. An image of the illuminated surface is captured and sheared to produce an interference or “speckle” pattern. Another speckle pattern is then captured while the surface of the test object is subject to a small load, for example, caused by vacuum, vibration, or heat. Because shearography detects out of plane deformation, it is particularly useful for inspecting layered materials for defects underlying the surface such as delamination. FIG. 1A depicts a layered material 100 formed of three layers 110, 120, and 130. A delamination 165 exists between layers 110 and 120. And, another delamination 175 exists between layers 120 and 130. When no load is applied to a top surface 101 of layered material 100, there is little or no out of plane deformation. In contrast, FIG. 1B depicts out of plane deformation 185 and 195 corresponding to delaminations 165 and 175, respectively, when top surface 101 of layered material 100 is subject to loading by vacuum, heat, or mechanical methods. As depicted in a shearographic image 199 in FIG. 1C, this method detects the out-of-plane deformation caused by delaminations 165 and 175 by subtracting a speckle pattern of top surface 101 of layered material 100 in the unloaded state from the speckle pattern of top surface 101 in the loaded state.
Materials used for interiors of aircraft and other vehicles are subject to demanding performance requirements including high strength to weight ratio, mechanical strength, and dimensional stability. Low heat, smoke, and toxin release in case of fire are also desirable. To meet these requirements, layered composite materials, such as multi-layered laminate materials, are often used. FIG. 1D shows an exemplary vehicle, aircraft 150 including an interior having a layered composite material, for example, as shown in FIG. 1A. In this case, layer 110 represents a common material used for the visible surfaces of aircraft interiors known as decorative laminate or “declam.” Declam generally includes one or more polymer layers and one or more adhesive layers. Declam typically resides over a panel skin, for example layer 120 in FIG. 1A, and is formed of one or more prepreg adhesive layers. The panel skin in-turn resides over a honeycomb core, for example layer 130, in FIG. 1A. Inspecting layered composite materials, such as declam covered areas, for defects during production of an aircraft and while the aircraft is in service presents several challenges including access to only the visible surfaces, large amounts of surfaces to inspect, and the desirability of a nondestructive method.
To date, use of shearography to inspect declam-covered areas has seen limited success due to the porous nature of the honeycomb core. Because defects can be the same size or smaller than the pores, conventional methods for shearographic inspection suffer from an inability to distinguish defects from noise caused by the porous materials (e.g., decorrelation noise or “D-Noise”). Thus, a need exists in the industry to address the aforementioned deficiencies and inadequacies.